1. Field
The present disclosure generally relates to acoustic treatments for reducing noise, and deals more particularly with septumization of honeycomb cellular cores.
2. Background
Aircraft engines produce noise primarily due to rotating blades which compress the airflow as well as rotating blades which extract energy from the airflow and the high-speed airflow through the exhaust nozzles. In order to reduce noise and satisfy noise regulations governing commercial aircraft, aircraft engines may incorporate acoustic panels in various parts of the engine, such as in the nacelle inlets, as well as the aft bypass duct and primary nozzle. These acoustic panels, sometimes referred to as acoustic treatments or acoustic liners, may comprise a honeycomb core sandwiched between a perforated inner skin and a non-perforated outer skin. The honeycomb core often has a middle layer of porous material called a septum which is used to increase the acoustic performance of the liner. The design parameters of the septum in the cells of the honeycomb core usually consist of the porosity of the layer as well as the depth or location relative to the perforated inner skin. The cavities that are formed by the septums act as Helmholtz resonators that attenuate the engine noise.
There are at least three known methods for septumization of honeycomb cores, each of which requires the septa to be installed in the core in a separate process after the honeycomb core has been fabricated. The first method requires splitting the core and using an adhesive to bond a septum layer between the split cores, however this approach is time consuming, labor intensive and may decrease the mechanical performance of the core because it requires splitting the core. The second method involves a lost wax process which creates a buried septum, where a honeycomb core is pressed into wax. A thin layer of liquid resin floats on top of the wax which is then cured to form a solid layer and the wax is melted out of the honeycomb core. This buried septum process requires an extra step in which a laser is used to perforate the solid septum layer to the desired porosity. The third method involves using an automated robotic process where individual septum pieces with tabs are inserted into each honeycomb cell. The tabs of the individual septum pieces are bonded to the honeycomb cell walls by carefully dipping the honeycomb panel to a certain depth into a liquid adhesive. The process of installing the individual septum pieces can be time consuming and expensive. Accordingly, there is a need for a method of septumizing a honeycomb core used in an acoustic sandwich structure which obviates the need for installing individual septum pieces in the core cells, or splitting the core to allow the installation of a septum layer, or the many steps in using a lost wax process to create a solid septum layer and then using a laser to perforate the solid septum layer.